The invention relates to tires and more specifically to the architecture of the tread portion of the tires.
The tread portion of tires usually comprises a tread, a belt reinforcing element comprising at least one and usually more than one layer of reinforcing cords, and a carcass. The tread portion comes in contact with the ground and needs to transmit the forces responsible for the handling of the vehicle. In this respect, it is necessary for the tread portion to be relatively rigid, while sufficiently flexible to achieve a good flattening when the tread comes in contact with the road.
To achieve a very good compromise between all the desired performances, it has become common, especially for performance tires for passenger cars, to have more and more different compounds designed for a specific role in a specific location in a tire. For instance, it is known technique to calender the reinforcing cords with rubber to manufacture plies in which the cords are embedded in rubber, the calendering rubber compound is selected to achieve a specific function such as adherence of rubber to a steel cord, and or to give to the tire unique properties.
It is known that the compounds that bind the reinforcing cords to the layers that reinforce the tread region play a significant role in obtaining a high rigidity of the tread portion. The calendering rubber compound of the belt cross plies is designed to have a high elastic modulus. By contrast, the calendering rubber compounds for the carcass cords usually have a low elastic modulus because they need to withstand the high amounts of deformation they experience in the sidewalls of the tires.
Furthermore, there is an endless need to reduce the fuel consumption of vehicles. To this end, it is desired to design tires which have a very low rolling resistance while keeping the other performances unaffected, namely wear resistance, adherence, handling, etc. This also has an influence on the design of the rubber compounds and of the reinforcing layers of cords and their locations in the tire.
Therefore, even when reinforcing cords can be directly applied rather than by precalendering them and making a semi-finished ply before building the tire with said semi-finished ply, the designer of the tire is likely to use the same rubber compounds on a given side of the ply at every location in the tire.
An on-going goal of this invention is to lower the overall mass of the tire in an attempt to decrease the rolling resistance of the tire, while keeping at a valuable level other performances of the tire.
A further objective of the invention is to provide a tire structure which readily lends itself to mechanized manufacture.
In the following specification, the term xe2x80x9ccordxe2x80x9d is understood to mean both monofilaments and multifilaments, or assemblies such as ply cords, twisted cords, or alternatively any kind of equivalent assembly, this being irrespective of the material and treatment of these cords which may, for example, have a surface treatment or be coated or pre-coated with adhesive to promote them to stick to the rubber. The term xe2x80x9creinforcing layerxe2x80x9d means cords arranged in parallel with respect to each other, generally characterized by the angle made by a cord at and with respect to the mid circumferential center plane CP perpendicular to the axis of the tire.
The term xe2x80x9ccompoundxe2x80x9d means a rubber mix having one or more base elastomer and additives selected in accordance with the properties of the compound desired for the zone of the tire it is used. xe2x80x9cContactxe2x80x9d between a cord and a rubber mix or compound is understood to mean the fact that at least some of the external face of the cord is in close contact with said compound. The xe2x80x9celastic modulusxe2x80x9d of a compound is understood to be a secant extension modulus obtained at a deformation in uniaxial extension of the order of 10% at room temperature.
The tire according to the invention comprises:
a tread,
at least one radially outermost reinforcing layer of belt reinforcement cords arranged substantially at a zero angle with respect to a mid circumferential center plane perpendicular to the axis of the tire,
a carcass having carcass cords and being located radially inwardly of the at least one radially outermost reinforcing layer of belt reinforcement textile cords, said carcass being anchored in each side of the tire in a bead,
an anchoring means for anchoring said carcass in the beads,
and the tire further having, in a substantial portion of the axial width of said at least one radially outermost reinforcing layer, in any cross section perpendicular to the axis of the tire and intersecting a cord of said at least one radially outermost reinforcing layer:
a tread compound having direct contact with the radially outward side of the cords of said at least one radially outermost reinforcing layer, and
a first decoupling layer comprising a decoupling compound different than the tread compound, said decoupling compound having direct contact with the radially inner side of the cords of said at least one radially outermost reinforcing layer, the tread compound having contact with the decoupling compound in interfaces terminating axially on the cords of said at least one radially outermost reinforcing layer.
In other words, the invention proposes to apply a tread compound directly on the outermost layer of reinforcing cords, preferably on textile cords, without using any calendering rubber. The tire of the invention features the direct contact of tread compound on textile cords. In many embodiments the compound used radially inwardly of said textile cords will be different in nature, the tire in this particular case will also feature interfaces between the different compounds. The path of said interfaces goes from cord to cord in the radially outermost reinforcing layer of belt reinforcement textile cords.
The invention proposes a tire further having a first decoupling layer of a decoupling compound different than the tread compound, said decoupling compound being in direct contact with the radially inner side of the textile cords of the radially outermost reinforcing layer of belt reinforcing textile cords, the tread compound being in contact with the decoupling compound in interfaces terminating axially on the textile cords of the radially outermost reinforcing layer of belt reinforcement textile cords.
As the tire of the invention does not have any calendering rubber on a substantial portion of the axial width of said at least one radially outermost reinforcing layer of belt reinforcement textile cords. the thickness of the compound in direct contact with the radially outward side of the textile cords of said at least one radially outermost reinforcing layer of belt reinforcing textile cords (which is a tread compound), measured in a cross section perpendicular to the axis of the tire and radially above an outward side of one said textile cord, is greater than the usual thickness of calendering rubber layers. It has been referred to a substantial portion of the axial width of said at least one radially outermost reinforcing layer to notify to the skilled person that the direct contact of the tread compound as explained has a significant technical feature. That means that the direct contact occurs at least along continuous portions axially on several adjacent cords. For instance a direct contact on all the textile cords included in a portion of at least 5% of the total width of the radially outermost reinforcing layer of belt reinforcement textile cords is significant.
The xe2x80x9cdecoupling compoundxe2x80x9d used can be the same in several decoupling layers, as it will become apparent in the disclosure of a preferred embodiment of the invention. In said preferred embodiment, it is also apparent that the bead reinforcement is obtained with a radially oriented pile of circumferential cords bordering laterally the carcass, although the invention is not restricted to that particular way to embody the anchoring means for anchoring the carcass.